وَهُوَ وَاحِدٌ
And it is one.
Here unity refers to God's attribute of speech. Eternal divine speech is not a collection of independent divine beings or a plurality of separately existing eternal attributes.
The existence of distinct revealed books does not entail that the attribute itself divides into many eternal personal entities. The concept of kalām nafsī clarifies the distinction between speech subsisting in God and the words heard or spoken by people.